feat: U3 @sealed optional segment marking - phase 2 complete

- UTSM_SEG_F_UNSEALED: plaintext direct-store segments (zero crypto overhead) while keeping capability checks, dirty tracking, intent/commit log and seqlock protocol; default remains sealed
- linker.ld: .sealed output section with __sealed_start/__sealed_end symbols for future static sealed data
- selftest: unsealed segment stores plaintext verifiably + read-back, full SELFTEST PASS (.build_tmp/qemu_serial_u3sealed.log)
- phase 2 (UTSM/UTRW) now complete in ROADMAP_INSANE

CODE/UTSM/utrw/read.c

@@ -20,6 +20,12 @@ int utsm_read(utsm_capability cap, u64 offset, void *dst, u64 len) {
         return status;
     }
 
+    /* U3: @unsealed 段 —— 明文直存，0 加解密开销（capability 校验照走） */
+    if (desc->flags & UTSM_SEG_F_UNSEALED) {
+        memcpy(dst, desc->cipher_base + offset, len);
+        return UTSM_OK;
+    }
+
     utsm_key_material key;
     status = utsm_pckc_get_or_derive(cap.segment_slot, desc, &key);
     if (status != UTSM_OK) {